Polychlorotrifluoroethylene [PCTFE] is known to have good gas barrier properties and low water vapor permeability. However, it has problems; it is insufficient in stress cracking resistance, thermal stability and chemical resistance and the temperature range within which it can be molded is narrow.
Attempts have been made to provide PCTFE with stress cracking resistance by copolymerizing chlorotrifluoroethylene [CTFE] with various modifier monomers. For example, CTFE/PAVE copolymers resulting from copolymerization of 0.01 to 1 mole percent of a perfluoro(alkyl vinyl ether) [PAVE] have been disclosed (cf. e.g. Patent Document 1: Japanese Kokai (Laid-open) Publication H03-287614).
These CTFE/PAVE copolymers have improved stress cracking resistance but are still insufficient in thermal stability. This is because their thermal decomposition temperature is not so high, so that, for example, in the case of coextrusion molding with a partner material having a high melting point, it is to be exposed to severe molding conditions and thus the partner materials that can be used in combination are unfavorably restricted.
Further, fluoroelastomers having a copolymerization composition comprising 30 to 60 mole percent of vinylidene fluoride, 10 to 40 mole percent of tetrafluoroethylene [TFE] and 10 to 30 mole percent of CTFE, optionally together with 5 to 15 mole percent of perfluoro(methyl vinyl ether) (cf. e.g. Patent Document 2: Japanese Kokai Publication 2000-7732) and hydroxyl group-containing fluorocopolymers resulting from polymerization of (a) TFE and/or CTFE, (b) a vinyl ether type monomer and (c) a hydroxyl group-containing vinyl ether type monomer in the proportion such that the ratio a/(b+c) amounts to about 40 to 60 mole percent (cf. e.g. Patent Document 3: Japanese Kokai Publication S60-88078), among others, have been proposed as CTFE copolymers. However, these CTFE copolymers still have a problem; namely, they are inferior in chemical resistance and thermal stability.
Other fluoropolymers have also been proposed as CTFE copolymers. They have a copolymerization composition comprising 50 to 99.8 mole percent of TFE and/or CTFE, 0.1 to 49.99 mole percent of a fluoromonomer other than TFE and CTFE and 0.01 to 5 mole percent of at least one compound selected from among itaconic acid, citraconic acid, and anhydrides thereof (cf. e.g. Patent Document 4: Laid-open European Patent Application No. 1375539). This fluoropolymer needs to have such compounds as itaconic acid as a copolymerization composition for improving adhesion strength. However, there is no specific disclosure about fluoropolymers containing both TFE and CTFE in the copolymerization composition thereof.